1. Field of the Invention
The present invention relates to an apparatus for and method of controlling a variable valve mechanism operated by an electric actuator and varying an open characteristic of an engine valve. More particularly, the invention relates to a technique for diagnosing whether or not a failure occurs in the variable valve mechanism, on the basis of a manipulated variable of the electric actuator.
2. Description of the Related Art
Japanese Laid-open (Kokai) Patent Application Publication No. H06(1994)-249040 discloses a controlling apparatus for a vehicle which controls a throttle actuator on the basis of an output of a throttle sensor, wherein when a state where drive current of the throttle actuator exceeds a threshold continues for a predetermined time or longer, it is determined that a failure occurs.
Often, in a variable valve mechanism for changing an open characteristic of an engine valve by using an electric actuator such as a motor, a duty ratio (manipulated variable) of a current supply control signal of the electric actuator may be set on the basis of a difference between a target open characteristic and an actual open characteristic.
In such a variable valve mechanism, when a movable part driven by the electric actuator becomes unmovable or the movement of the movable part becomes extremely slow due to a failure, a control error does not decrease, so that the duty ratio is set to 100% and held in this state.
Consequently, it is able to estimate the occurrence of a failure in the variable valve mechanism based on a length of duration of the state where the duty ratio is held at 100%.
However, in the state where the duty ratio is 100%, the heat generation amount of the electric actuator and a drive circuit of the electric actuator is large. If this state is left for a long time, a failure might further occur in the normal electric actuator and the normal drive circuit.
For this reason, it is desired to shorten a criterion time in the case of performing an abnormal diagnosis on the basis of the continuation period and to determine a failure early. However, the duty ration can be set to 100% even when the mechanism is normal. If the criterion time is too short, an abnormality in the variable valve mechanism might be erroneously diagnosed.